<!DOCTYPE html>
<html lang="zh-CN">
    <head>
        <meta charset="UTF-8" />
        <title>09_CSS3-基本介绍</title>
        <style>
            div {
                width: 200px;
                height: 200px;
                background-color: aqua;
                border-radius: 20px;
            }
        </style>
    </head>
    <body>
        <div></div>
    </body>
</html>